1. Field of the Invention
The present invention relates generally to a system for, and method of, processing image data for an output device such as a printer or display, wherein colors in a source image are assigned to a limited number of colors available in the printer or display (i.e., printable colors) in order to print or display full-color or other multicolor image data using fewer colors than are in the source image. The system and method of this invention are particularly compatible with a color printer capable of printing only a limited number of colors. This type of printer is referred to herein as a “limited-color printer.”
2. Description of the Related Art
By using 256 levels each for red, green, and blue, modern graphics applications can express more than 16 million colors in a full-color image. Full-color printers capable of printing these images are now commonly available. While printers used in business have primarily been monochrome (black & white) printers because of their greater speed and economy, color printers are now also beginning to be used in business settings.
While full-color printers make it possible to use a palette of diverse colors for greater expressiveness and variety, the drawback is that the more colors that are printed, the longer it takes and the higher the operating costs. This is a particular problem with printers used in business settings where high-speed printing is essential. Balancing the desire for the expressiveness that is possible with color printing with the need for fast printing and economical operation is particularly important with printers used for specialized applications such as POS terminals. In such applications, there is a 1:1 relationship between a customer and the printing operation, printer output is handed directly to the customer on the spot, and high speed printing is therefore essential. Two-color printers, that is, printers limited to printing two colors, are therefore beginning to be used for such specialized printing applications. Applications for this type of printer currently include POS printers such as used in the retail industry, ATM printers used in banks, printers for printing tickets for parking lots and customer service numbers, and printers used in kiosk terminals in convenience stores, for example. The primary object of such printers is to print text. Text data is typically printed in black, and the two-color printers used in such applications are typically capable of printing black and one other color (normally red, green, or blue). The printer could, however, print two colors neither of which is black, or the printer could be a three-color printer capable of printing two colors in addition to black.
In order to print a full-color image on this type of limited-color printer, the data representing the full-color image must be reduced, through a process known as color reduction, to the colors that can be printed by the printer. In addition to enabling printing full-color images on a limited color printer, color reduction of the image data is also used to, for example, print an image using just the available remaining colors when a particular color of ink runs out. The data that represent the source image to which such color reduction is applied and to which color colors are assigned (color assignment) is referred to herein as “source data.”
Methods for reducing the colors in a full-color or other multicolor image include simple color reduction, dithering, and error diffusion. Dithering and error diffusion, however, introduce a fine noise pattern as a result of the color reduction process. When an image with such noise spots is then printed with only a few colors, the spots become obvious and conspicuous. It is particularly important for logos printed on a receipt by a POS printer to be clearly defined so that the printed image is easily recognizable. The contours and shape of the logo must therefore be clearly expressed even when printed with a small palette of colors. Some logos are designed with a complicated blend of colors and color brightness (appearing as a difference in luminance (brightness) during image data processing).
Simple color reduction techniques simply digitize RGB color depth referenced to a defined median value, thus emphasizing the color difference and not clearly reflecting the difference in brightness. The result is that when logos with complicated coloring and subtle brightness (luminance) differences are reduced with simple color reduction, the loss of luminance difference information results in the printed logo differing greatly from the original logo.
Furthermore, when a logo for a particular printing purpose, such as a customer service notice, product advertisement, or coupon, is printed using only a few colors, it is desirable to be able to select among a variety of image processes in order to create the best possible print image.